1. Field of Invention
The present invention relates to HASE rheological additives for aqueous systems and emulsions. More particularly, the present invention relates to a pH-responsive composition and a method useful for modifying the rheological properties of aqueous emulsions.
2. Description of Related Art
Rheological additives are chemical compositions, which, added even in small amounts, modify a liquid system's rheological properties, such as viscosity and response to shear. Such additives or thickeners may be used in a variety of liquid systems including aqueous systems such as paints, aqueous inks, and personal care products. The additives improve the rheological properties by also affecting the dispersion, suspension and emulsification of pigments, binders and other solids within a vehicle.
Conventional rheological additives include those of natural and synthetic origin. Useful natural thickeners include, for example, guar gum, pectin, xanthan gum, and alginate. Useful synthetic thickeners include, for example, hydrophobically modified ethoxylated urethanes (HEUR), ethylcellulose methylcellulose, hydroxyethylcellulose (HEC), hydroxymethylcellulose, carboxymethylcellulose, and other modified cellulosics.
Conventional natural and synthetic polymers have limitations with respect to use as thickeners in aqueous systems, particularly in paints and coatings. In general, they do not provide a rheological profile suitable for adequate viscosity control, splatter resistance and flow properties required in paints and coatings. For example, HEC swells rapidly in water and forms lumps, which are not readily dispersible.
Hydrophobically modified alkali soluble emulsion (HASE) polymer systems are commonly employed to modify the rheological properties of aqueous emulsion systems. Under the influence of a base, organic or inorganic, the HASE particles gradually swell and expand to form a three-dimensional network by intermolecular hydrophobic aggregation between HASE polymer chains and/or with components of the emulsion. This network, combined with the hydrodynamic exclusion volume created by the expanded HASE chains, produces the desired thickening effect. This network being sensitive to applied stress, breaks down under shear and recovers when the stress is relieved. Such rheological properties are particularly desirable for paints and coatings because they make the formulation easy to apply onto a surface while providing the thickness needed for uniform coverage and avoid spattering.
HASE polymer systems can be prepared from the following monomers: (a) an ethylenically unsaturated carboxylic acid, (b) a nonionic ethylenically unsaturated monomer, and (c) an ethylenically unsaturated hydrophobic monomer. Representative HASE polymer systems include those shown in EP 226097 B1, EP 705852 B1, U.S. Pat. No. 4,384,096, and U.S. Pat. No. 5,874,495.
Therefore, polymeric thickeners are desired in the industry as an alternative for HEC and similar cellulosics. Such paints should be non-biodegradable, free of toxic by-products, economical to manufacture, and exhibit excellent storage stability.
It would be desirable to have a HASE system wherein rheological properties are improved. In particular, it would be desirable to have a HASE system that provides enhanced rheological properties in paints and coatings. Further in particular, it would be desirable to have a HASE system that provides improved viscosity control, splatter resistance and flow properties in paints and coatings.